iMac with Retina 5K Display 2014/2015 Memory Upgrades
NEMIX RAM 16GB Kit (2x8GB Upgrade) DDR3-1600 SODIMM PC3-12800 for Apple MacBook Pro Mid 2012 / iMac Late 2012 / iMac 2013, Late 2014, Mid 2015, Mac Mini Late 2012 at Amazon.com
Install memory in an iMac - Apple Support
iMac RAM for Model 13.2 to 15.1| DDR3-1600MHz | 8GBx4 | MacMemory.com
CMS 32GB (4X8GB) DDR3 12800 1600MHz Non ECC SODIMM Memory Ram Upgrade Compatible with Apple® iMac Retina 5K 27-Inch (Late 2014) Core I5/I7 - A6 at Amazon.com
Upgrade RAM on Apple Retina iMac 5K (Late 2015) / How To install memory - YouTube
OWC 16GB (2x8GB) ram Upgrade PC3-12800 DDR3L 1600MHz sodimm 204 Pin CL11 Memory Upgrade Kit Compatible with Late iMac, Mac Mini, and MacBook Pro Computer at Amazon.com
Memory in New $1099 iMac is Soldered and Not Upgradable - MacRumors
Install memory in an iMac - Apple Support
A-Tech 32GB Kit (4x8GB) RAM for Apple iMac (Late 2012, Late 2013, Late 2014, Mid 2015) | DDR3 1600MHz SODIMM PC3-12800 204-Pin SO-DIMM Max Memory Upgrade at Amazon.com
Memory in New $1099 iMac is Soldered and Not Upgradable - MacRumors